1. Field of the Invention
This invention relates generally to the use of a syngas or synthesis gas produced during a partial oxidation gasification process as the feedstock in a process for the direct reduction of iron also referred to as the "DRI" process.
2. Description of the Prior Art
Fuelled by an anticipated shortage of scrap and an increase in the number of steel mini-mills as alternatives to integrated steel plants, DRI production is currently undergoing a major expansion internationally. Solid feed based DRI processes can essentially be divided into two broad technologies according to the iron ore feed, that is, lump-and/or pellet-based or fines-based feed. Further sub-division can be made according to the hydrocarbon feedstock used, for example coal or gas.
The principal lump/pellet-based technologies are known as "Midrex" and "HYL III," and the fines-based technology is known as the "Fior" process. All employ reformed natural gas as the reducing gas feedstock for the DRI reaction. The HYL III and Fior processes use conventional steam reforming of natural gas to produce a synthesis gas including hydrogen and carbon monoxide. Impurities such as carbon dioxide are removed by scrubbers. The Midrex process employs a combination of steam and carbon dioxide reforming of natural gas in a proprietary reformer.
Given these modes of producing the reducing gas, the most favorable location for a DRI plant is where inexpensive natural gas and high grade iron ore are located in close proximity, since this obviates excessive transportation costs.
Partial oxidation gasification processes are often used to produce a gas comprising hydrogen and carbon monoxide as the major components. This gas is generally referred to as synthesis gas or "syngas." Such partial oxidation gasification processes are most effective when operated at relatively high pressures, generally greater than 20 atmospheres. As used herein, a partial oxidation reactor can also be referred to as a "gasification reactor," or simply a "gasifier" and these terms are often used equivalently and interchangeably.
The feedstock for a partial oxidation reaction is usually a hydrocarbonaceous material, that is, one or more materials, generally organic, which provide a source of hydrogen and carbon for the gasification reaction. The hydrocarbonaceous material can be in a gaseous, liquid or solid state, or in a combination as desired, for example, a solid-liquid composition in a fluidized state. Petroleum based feedstocks include petroleum coke, coal residual oils and byproducts from heavy crude oils. The coal or coke can be in a finely divided state. Waste plastic materials can also be used as the feedstock.
Many uses for the syngas produced from the partial oxidation reaction are at relatively lower pressures. Therefore, expansion of the high pressure syngas across a power recovery machine to obtain a syngas at reduced pressure is often employed. This type of expansion is often used as a means for generating electricity. The power generation step is not 100% efficient and some energy is lost in converting the energy to electricity. The electricity generated in such an expansion process requires step up transformers, additional electrical switchgear, and a use for the electricity.
The DRI process can use syngas as a feedstock, generally fed to the reaction chamber at a relatively low pressure, typically less than about 5 atmospheres for a moving bed reactor, and less than about 15 atmospheres for a fluidized bed reactor. The DRI offgas is cooled, compressed, sent to a carbon dioxide removal step, and then mixed with fresh syngas feed and recycled to the DRI process. The recycle compressor is a large energy user and often uses an electrically driven motor. This electrically driven motor is not 100% efficient and some energy is lost in converting electricity to shaft power. Use of an electrical motor requires step down transformers, additional electrical switchgear, and a source of electricity. Alternately, a steam driver can be used with similar energy losses and supporting equipment.
The operation of gasifiers at relatively high pressures in power generation systems, for example, the integrated gasification combined cycle (IGCC) system is disclosed in U.S. Pat. Nos. 5,117,623 and 5,345,756, wherein these systems are coupled with expanders, gas turbines and steam turbines for power generation. U.S. Pat. Nos. 5,531,424 and 5,370,727 disclose processes for the direct reduction of iron.